This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-294748, filed Oct. 18, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a surface acoustic wave apparatus having an interdigital transducer formed on a piezoelectric substrate.
A surface acoustic wave apparatus is mainly used as an intermediate frequency filter for electronic devices. These devices include television sets, communication devices, and cellular phones (of, for example, the CDMA system), etc. The surface acoustic wave apparatus is characterized in that it is compact and lightweight, and thus can be utilized to the full when utilized in a cellular phone.
Low loss, a narrow bandwidth and a steep cut-off frequency are demanded by the intermediate frequency filter for use in a cellular phone. As a filter serving as an intermediate frequency filter of this type, a surface acoustic wave apparatus having an interdigital transducer (IDT) as a main structural element has been developed.
In this surface acoustic wave apparatus, to meet the demands of narrow bandwidth and steep cut-off frequency characteristics, a piezoelectric substrate is used which is formed of a material, such as crystal, having characteristics of low fluctuation in oscillation irrespective of temperature changes.
In the surface acoustic wave apparatus, it is known that an internal reflected wave (a reflected acoustic wave, a reflected electric wave) occurs since the apparatus utilizes a mechanical vibration such as a surface acoustic wave (SAW). The internal reflected wave adversely influences the fundamental wave of the surface acoustic wave, thereby causing its amplitude attenuation or phase distortion, etc. To reduce the influence of the reflected wave upon the fundamental wave and to adjust the direction of transmission of the fundamental wave to a predetermined direction, a technique for adjusting or trimming the width of an electrode finger in the IDT section of the surface acoustic wave apparatus has been developed. This technique is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 54-17647.
In the disclosed apparatus, the adverse influence of the internal reflected wave upon the fundamental wave is not completely removed. In view of the transmission function characteristic of a signal passing band in the surface acoustic wave apparatus, the adverse influence appears as a characteristic curve asymmetric with respect to the basic frequency at the center of the curve, or as a distorted characteristic curve.
A description will now be given of the phase relationship between the exited wave and the reflected wave in the surface acoustic wave apparatus, which the inventors of this invention have especially focused on.
FIGS. 1A and FIG. 1C show cross sections of essential parts of the surface acoustic wave apparatus. The electrode fingers of the surface acoustic wave apparatus include pairs of electrode fingers of a narrow width of xcex/16, and pairs of electrode fingers of a wide width of 3xcex/16. Further, a plurality of pairs of electrode fingers projecting from a first common electrode and a plurality of pairs of electrode fingers projecting from a second common electrode are alternately arranged. The interval between each pair of adjacent electrode fingers is set at xcex/8.
FIG. 1B shows the phase relationship between the excited wave and the internal reflected wave that advances to the left in the figure, while FIG. 1D shows the phase relationship between the excited wave and the internal reflected wave that advances to the right in the figure. Suppose that the phase of the excited wave is a reference value, and the clockwise direction with respect to the phase of the vector of the excited wave is the direction of a phase delay in the reflected wave.
Referring first to FIGS. 1A and 1B, the relationship between the reflected wave that advances to the left in the figures, and the excited wave will be described.
Suppose that P1 represents any randomly-selected excitation point at which the surface acoustic wave is excited, A represents a reflected wave reflected from one EA of the edges of an electrode finger of a width (xcex/16) closest to the excitation point, and B represents a reflected wave reflected from the other edge EB of the electrode finger. Further, suppose that C represents a wave from the excitation point as a reference point, which is passed through the electrode finger closest to the excitation point, and reflected from one EC of the edges of an electrode finger of a width of 3xcex/16 adjacent to the first-mentioned electrode finger, and D represents a reflected wave reflected from the other edge ED of the second-mentioned electrode finger.
Concerning the reflected wave A, a phase delay element equal to a distance of 0.125xcex with respect to the phase of the excited wave at the excitation point occurs, and hence a phase delay of 45xc2x0 occurs with respect to the phase of the excited wave. In other words, supposing that a delay =X, X=45xc2x0 is given by the following equation:
(({fraction (0.125/2)})xc3x972)xcex:X=xcex:360xc2x0
(xc3x972) included in this equation indicates a coefficient for obtaining a double distance.
Further, concerning the reflected wave B, a phase delay element equal to a distance of (0.125+(xe2x85x9)) xcex with respect to the phase of the excited wave at the excitation point P1 occurs, and phase inversion ((xc2xd)xcex) occurs at the edge EB. Accordingly, a phase delay of 270xc2x0 occurs with respect to the phase of the excited wave. In other words, supposing that a delay=X, X=360xc3x97(0.125+(xe2x85x9)+(xc2xd))=270xc2x0 is given by
{(({fraction (0.125/2)})xc3x972)+(({fraction (1/16)})xc3x972)+(xc2xd)}xcex:X=xcex:360
(xc3x972) included in this equation indicates a coefficient for obtaining a double distance, and (xc2xd) indicates the amount of phase inversion at the edge EB.
Concerning the reflected wave C, a phase delay element equal to a distance of (0.125+(xe2x85x9)+(xc2xc)) xcex with respect to the phase of the excited wave at the excitation point P1 occurs, and hence a phase delay of 180xc2x0 occurs with respect to the phase of the excited wave. In other words, supposing that a delay=X, X=360xc3x97(0.125+(xe2x85x9)+(xc2xc))=180xc2x0 is given by
{(({fraction (0.125/2)})xc3x972)+(({fraction (1/16)})xc3x972)+((xe2x85x9)xc3x972)}xcex:X=xcex:360
(xc3x972) included in this equation indicates a coefficient for obtaining a double distance.
Concerning the reflected wave D, a phase delay element equal to a distance of (0.125+(xe2x85x9)+(xc2xc)+(xe2x85x9c)) xcex with respect to the phase of the excited wave at the excitation point P1 occurs, and phase inversion occurs at the edge ED. Accordingly, a phase delay of 135xc2x0 occurs with respect to the phase of the excited wave. In other words, supposing that a delay=X, X=360xc3x97((xe2x85x9)+(xe2x85x9)+(xc2xc)+(xe2x85x9c)+(xc2xd))=495xc2x0=135xc2x0 is given by
{(({fraction (0.125/2)})xc3x972)+(({fraction (1/16)})xc3x972)+((xe2x85x9)xc3x972)+({fraction (3/16)})xc3x972+(xc2xd)}xcex:X=xcex:360
(xc3x972) included in this equation indicates a coefficient for obtaining a double distance, and (xc2xd) indicates the amount of phase inversion at the edge ED.
The result of synthesizing the vectors of the reflected waves A, B, C and D corresponds to the phase delay amount of the internal reflected wave relative to the excited wave. As shown in FIG. 1B, the phase of the vector obtained by synthesizing the vectors of the internal reflected waves A, B, C and D is 157.5xc2x0. This value indicates that the phase is deviated by 22.5xc2x0 from a phase (180xc2x0) in a direction in which the excited wave is offset.
The synthesized vector is obtained by synthesizing the x- and y-components of the vectors of the reflected waves A, B, C and D in the orthogonal coordinates.
Referring then to FIGS. 1C and 1D, the relationship between the reflected wave that advances to the right in the figures and the excited wave will be explained.
Suppose that P2 represents any randomly-selected excitation point at which the surface acoustic wave is excited, E represents a reflected wave reflected from one EE of the edges of an electrode finger of a width (3xcex/16) closest to the excitation point, and F represents a reflected wave reflected from the other edge EF of the electrode finger. Further, suppose that G represents a wave from the excitation point as a reference point, which is passed through the electrode finger closest to the excitation point, and reflected from one EG of the edges of an electrode finger of a width of xcex/16 adjacent to the first-mentioned electrode finger, and H represents a reflected wave reflected from the other edge EH of the second-mentioned electrode finger.
Concerning the reflected wave E, a phase delay element equal to a distance of 0.125xcex with respect to the phase of the excited wave at the excitation point P2 occurs, and hence a phase delay of 45xc2x0 occurs with respect to the phase of the excited wave.
Further, concerning the reflected wave F, a phase delay element equal to a distance of (0.125+(xe2x85x9c)) xcex with respect to the phase of the excited wave at the excitation point P2 occurs, and phase inversion occurs at the edge EF. Accordingly, a phase delay of 0xc2x0 occurs with respect to the phase of the excited wave.
Concerning the reflected wave G, a phase delay element equal to a distance of (0.125+(xe2x85x9c)+0.25) xcex with respect to the phase of the excited wave at the excitation point P2 occurs, and hence a phase delay of 270xc2x0 occurs with respect to the phase of the excited wave.
Concerning the reflected wave H, a phase delay element equal to a distance of (0.125+(xe2x85x9c)+0.25+(xe2x85x9)) xcex with respect to the phase of the excited wave at the excitation point P2 occurs, and phase inversion occurs at the edge EH. Accordingly, a phase delay of 190xc2x0 occurs with respect to the phase of the excited wave.
The above-described phase delays can be given by the same equations as used when explaining FIGS. 1A and 1B.
The result of synthesizing the vectors of the reflected waves E, F, G and H corresponds to the phase delay amount of the internal reflected wave relative to the excited wave. As shown in FIG. 1D, the phase of the vector obtained by synthesizing the vectors of the internal reflected waves E, F, G and H is 22.5xc2x0. This value indicates that the phase is deviated by 22.5xc2x0 from a phase (0xc2x0) in the same direction as that of the excited wave.
It is understood from the above-described results that the excited wave (fundamental wave) is not transmitted in a desired direction with a sufficiently high coefficient because of the influence of the reflected wave.
It is the object of the invention to provide a surface acoustic wave apparatus that shows a much better transmission function characteristic in its signal passing band, and a low insertion loss.
To attain the object, the following apparatus is provided:
(A) A surface acoustic wave apparatus equipped with an interdigital transducer that includes first and second common electrodes formed in parallel on a piezoelectric substrate, a first split electrode group connected to the first common electrode and extending toward the second common electrode, and a second split electrode group connected to the second common electrode and extending toward the first common electrode; characterized in that:
a plurality of pairs of electrode fingers that constitute the first split electrode group, and a plurality of pairs of electrode fingers that constitute the second split electrode group are interleaved at a cycle of xcex/2;
each pair of electrode fingers consist of a narrow-width electrode finger narrower than xcex/8 (xcex indicates a wavelength of a surface acoustic wave serving as an operation central frequency), and a wide-width electrode finger wider than xcex/8; and
a synthesized vector, obtained by synthesizing vectors of waves reflected from electrode fingers that are included within a distance of xcex/2 from any randomly-selected excitation point in a surface wave propagation direction, has a phase xcfx86 expressed by nxcfx80xe2x88x925xc2x0xe2x89xa6xcfx86xe2x89xa6nxcfx80+5xc2x0 (n: a natural number) with respect to a phase of a wave excited by the interdigital transducer.
(B) The surface acoustic wave apparatus of the present invention described in item (A) has a structure in which a distance (t1) between centers of a randomly-selected electrode finger and one of the electrode fingers located at the opposite sides of the randomly-selected electrode finger, differs from a distance (t2) between centers of the randomly-selected electrode finger and the other of the electrode fingers located at the opposite sides of the randomly-selected electrode finger.
(C) The surface acoustic wave apparatus of the present invention described in item (A) is characterized in that said each pair of electrode fingers consist of a narrow-width electrode finger having a width of xcex/16, and a wide-width electrode finger having a width w of 0.1465xcexxe2x89xa6wxe2x89xa60.1605xcex.
(D) A surface acoustic wave apparatus according to another aspect of the present invention is characterized in that: a plurality of pairs of electrode fingers that constitute the first split electrode group, and a plurality of pairs of electrode fingers that constitute the second split electrode group are interleaved; each pair of electrode fingers consist of a narrow-width electrode finger narrower than xcex/8 (xcex indicates a wavelength of a surface acoustic wave serving as an operation central frequency), and a wide-width electrode finger wider than xcex/8; a sum of the widths of the narrow-width electrode finger and the wide-width electrode finger is lower than xcex/4; and a sum of the widths of the narrow-width electrode finger and the wide-width electrode finger and a distance between adjacent edges of these electrode fingers is 3xcex/8.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.